1. Field of the Invention
The present invention relates to an image forming method of forming an image on a recording material by electrophotography.
2. Description of the Prior Art
In an image forming apparatus using the electrophotographic or electrostatic recording method, an electrostatic latent image if formed on an image retainer and is developed with charged particle or toner. This apparatus is produced as a reproducing machine or printer. In order to form a multi-color image or a composed image (in which a plurality of documents or image information and a document image are superposed), the aforementioned principle is utilized in the following manner. More specifically, one cycle of (1) charging, (2) image exposure and (3) development is performed twice on the image retainer having a photoconductive layer on a conductive substrate (as is disclosed in Japanese Patent Application No. 58-184381, for example). As an alternative, there is a method of performing twice one cycle of (1) primary charging, (2) secondary charging and image exposure, (3) uniform exposure and (4) development or a method of performing twice one cycle of (1) primary charging, (2) secondary charging, (3) image exposure and (4) development (as is disclosed in Japanese Patent Application No. 58-183152), for example) by using an image retainer in which a transparent insulating layer is formed outside of a photoconductive layer. Any of these methods makes possible a multi-color development or image composition. Since the superposed image can be transferred to a transfer member by a single transfer process, the apparatus for forming the multi-color or composed image can be realized with a simple structure. A developing method therefor is required to perform the development by using a developer composed of a mixture of a nonmagnetic toner and a magnetic carrier, for example, under the conditions specified in Japanese Patent Application Nos. 58-57446 or 60-192712. This developing method belongs to a kind of magnetic brush developing method and is characterized in that the magnetic brush is not brought into contact with the image retainer, but only the toner is flown onto the latent image surface of the image retainer.
In one example of the above-specified image forming apparatus, latent images of different colors are formed by latent image forming means and are developed with toners of corresponding colors.
This multi-color image forming apparatus is represented by an apparatus in which an image retainer (which may hereinafter be called a "photosensitive member") having a photoconductive substance on a conductive substrate is irradiated with an optical beam of a laser or the like to form an electrostatic latent image. In this apparatus, the multi-color image is formed in accordance with the flow chart of FIG. 4.
FIG. 4 illustrates the variations in the surface potential of the image retainer. In FIG. 4: reference letters PH denote the exposed portion of the image retainer; letters DA denote the unexposed portion of the image retainer; letters T.sub.1 denote the toner deposited onto the image retainer by a first development; letters T.sub.2 denote the toner deposited to the image retainer by a second development; and letters DUP denote the rise of the potential, which has been caused by the deposition of toner T.sub.1 to the exposed portion PH by the first development. For purpose of description, the polarity of the latent image is assumed to be positive.
(1) The image retainer is uniformly charged by a charging device to attain a constant positive surface potential E.
(2) A first image exposure is applied with an exposure light source such as a laser, a cathode ray tube or an LED so that the potential of the exposed portion PH drops in accordance with the amount of light.
(3) The electrostatic latent image thus formed is developed by a developing device to which is applied a positive bias substantially equal to the surface potential E of the unexposed portion. As a result, the positively charged toner T.sub.1 is deposited to the exposed portion PH having a relatively lower potential to form a first toner image. The region provided with this toner image has its potential raised by DUP as a result of the deposition of the positively charged toner T.sub.1 but not usually to the same potential as that of the unexposed portion DA.
(4) Next, the surface of the image retainer provided with the first toner image is subjected to a second charging operation to attain a uniform surface potential E no matter whether the toner T.sub.1 is present or not.
(5) The surface of this image retainer is subjected to a second image exposure to form an electrostatic latent image.
(6) This latent image is developed like the foregoing operation (3) with the positively charged toner T.sub.2 in a color different from that of the toner T.sub.1 to form a second toner image.
Similar processes are accomplished a desired number of times to form the multi-color image on the image retainer. This multi-color toner image is transferred to the transfer material and is fixed by heat or under pressure to attain a multi-color recorded image. In this case, the toner and charges residing on the surface of the image retainer are cleaned so that they may be used for forming a subsequent multi-color image.
The above-specified process can be applied not only to the multi-color image but also to an apparatus for forming a recorded image by composing toner images on an image retainer and transferring them as a whole.
The following two methods exist in case various colors are to be expressed by the methods described above:
(1) The method in which toners of different colors are not directly superposed; and
(2) The method in which toners of different colors are superposed.
In the method (1), colors are generated (e.g., additive mixture of colors) in a dummy manner on the recording paper by not superposing but distributing on principle the multi-color toners T.sub.1 and T.sub.2 on the image retainer 1, as shown in FIG. 5A.
In the method (2), colors are generated (e.g., subtractive mixture of colors) by developing tones of different colors in a superposed manner on a toner image of a certain color, as shown in FIG. 5B.
The color reproductivity of the methods (1) and (2) usually become different even if a common toner is used. As a matter of fact, a method having the methods (1) and (2) in a compatible manner is frequently adopted because the color reproducing range can be widened to reproduce many colors.
Incidentally, if an image exposure light is absorbed when a toner image of the toner T.sub.1 formed on an image retainer is irradiated with the image exposure light, the photoconductive layer remains in the insulated state so that its potential will not drop. Then, the toner T.sub.2 having been developed later becomes reluctant to be deposited on that position, as shown in FIG. 5D. As a result, the color reproduced region resorting to the method (2) is highly distorted, and the color reproductivity according to the method (1) is troubled, as shown in FIG. 5C, if the positions of the images of the individual colors fail to be strictly registered.
The description thus far made corresponds to the case of inverted development. If the image exposure light is absorbed by the toner T.sub.1 when a normal development is to be accomplished, the succeeding toner T.sub.2 will in turn be deposited on the preceding toner T.sub.1 in an unselective manner to cause turbidity of colors.
In order to avoid this problem, there has been proposed a method (as is disclosed in Japanese Patent Application Nos. 59-181087 or 59-181550), in which the yellow and magenta toners are developed prior to the other toners by using a laser beam of near infrared rays as the image exposure means. According to this method, the yellow image underlies another color on the image retainer but overlies another color on the transfer material. Incidentally, the yellow has a higher surface reflection than those of other colors so that the multi-color image obtained by the above-specified method has its yellow color emphasized more than necessary especially in the colors having yellow in addition to green and red. This raises a problem that the colors are remarkably difficult to control. This problem leads to a serious trouble especially in case a black color is to be expressed with the yellow, magenta and cyan toners. It is, therefore, preferable to use a special toner for expressing the black color.
Incidentally, the black toner using carbon black according to the prior art has such a wide absorption wavelength range as to substantially absorb not only a visible light but also most lights to which the photoconductive layer of the image retainer is sensitive. If the development with the black toner is accomplished prior to those with the yellow, magenta and cyan toners in case a multi-color image is to be formed with the other toners, the toners of the other colors are not developed in the positions where the Black toner is applied, as has been described hereinbefore. As a result, anything but a color having low lightness and saturation can be reproduced. If, on the contrary, the development with the black toner is accomplished after those with the other toners, the contrast of the latent image potential drops to make it reluctant to deposit the black toner. This in turn drops the black density to make the letter portions obscure and make the shades reluctant to appear in intermediate color portions.
It is certified in the experiments that the transfer efficiency is increased and the transfer material can be separated more easily from the image retainer if the image retainer is subjected to a uniform exposure prior to the transfer of the toner image to the transfer material in said processes (an exposure before transfer).
The above processes are carried out in the reversal development. Direct after the development, the electric potential at the environment of the portion on the image retainer on which the toner is attached is high, but the electric potential is lowered when it is subjected to the exposure before transfer. The exposure before transfer is, however, absorbed to a large extent by the toner at the portion where the toner is attached, so that the electric potential is not lowered sufficiently. As a result, the surface potential at this environment becomes as shown in FIG. 11.
Under such circumstances, a part of the toner T forming the toner image is separated from the original position and flown in the environment or floated in the apparatus, thereby causing the inside of the apparatus being soiled.
The above phenomenon depends on the fact that the toner T receives a strong electrostatic force in a direction parallel to the surface of the image retainer as shown in FIG. 12. Arrows show lines of electrostatic force and the positively charged toner T attached on the photoconductive layer 12 receives forces in the direction of the electrostatic force.
Such a phenomenon that a part of the toner T is separated from the original position and flown in the environment causes the image to be blurred and deteriorated in quality because the end portions or thin lines of the image become vague and the noise is formed in the screen image. Further, the soil of inside of the apparatus causes a bad influence just on the operation of the apparatus and the trouble and the stain of the image.
In the multi-color image formation or the superposition of the toner images, toner is attached on the image retainer in the form of multi-layers. However, the more the distance between the position of the toner and the surface of the image retainer the more easily the separation of the toner from the surface of the image retainer will be. As stated above, the image is deteriorated, the apparatus is soiled and th recording paper is stained by moving the toner on the image retainer to the another position by a little cause, such as electrical, optical or mechanical external forces.
Further, in said process, a portion where the toner on the image retainer is attached is subjected to each step of charging.fwdarw.image exposing.fwdarw.developing.fwdarw.charging.fwdarw. . . . , and a portion where the toner is not attached is subjected to the charging step repeatedly. Accordingly, the electric potential at the charging start time in the charging steps after the first charging step is varied due to the fact whether the toner is attached or not, or what color toner is attached. Accordingly, the surface potential of the image retainer becomes uneven, so that the toner or carrier is attached on the non-image portion or image portion on the image retainer, thereby causing the image noise or color turbidity.
Further, it is inclined that the surface potential at the previously exposed portion becomes lower than that at the portion not exposed, because of the memory effect of the photoconductive layer on the image retainer.
As stated above, the surface potential is varied according to the hysteresis of the position. Such a method of uniformly exposing before charging may be considered to avoid the variation, such exposure light is absorbed by the toner on the image retainer, so that a sufficient effect can not be expected and the image becomes vague due to the state as shown in FIG. 12.
As a method of cleaning off the toner left on the image retainer after the aforementioned image forming process, on the other hand, the method bringing a cleaning blade or a fur brush into contact with the image retainer to mechanically scrape off the toner left after the transfer is the most effective and is generally used.
Incidentally, especially in case that process is accomplished in the reversal development, the surroundings around the positions of the image retainer, to which the toners have been deposited, take higher potentials, which are highly dispersed depending upon the positions, after the transfer. As a result, an electrostatic force acts between the image retainer and the toners left after the transfer so that it restricts the toners to invite an insufficient cleaning effect. This contaminates the inside of the reproducing machine and exerts serious adverse effects upon a next image to be formed. At the charging step, more specifically, an even potential is reluctant to establish. The latent image is disturbed at the image exposure step. The toner image is blotted at the development step. The transfer is partially missing at the transfer step. Thus, the toners left uncleaned will be accumulated to cause the above-specified phenomena more.
In order to solve this problem, there is known the method of applying a uniform exposure before the cleaning step. According to this method, the charges residing on the image retainer after the transfer are eliminated to release the restrictions of the toners. If, however, the uniform exposure light is shielded by the toners, the tential difference increases between the portions where the toners are deposited and not. This shielding effect of the toners is prominent for the black toner of higher density.
Since the portions having the toners will have their potentials retained, the effect of the uniform exposure is lost to leave the insufficient cleaning unsolved. On the other hand, the portions having no toner will have their potentials drop, and the surface potential in the neighborhood is illustrated in FIG. 11.
Accordingly, it results in problems similar that in said exposure before transfer and said exposure before charging.